For the comfort of occupants of the cabin of a motor vehicle when they require different air temperatures in the cabin, some vehicles have installations of the above kind which comprise means defining separate left and right hand circuits for the processing of the air which is to be delivered into the left and right hand halves of the cabin. In such an installation, independent adjustment is possible, as between the left hand half of the vehicle and its right hand half, of the temperature and other parameters. If the separate treatment circuits were to extend up to the slots that are provided for the purposes of de-icing or de-emisting of the windshield, the difference in temperature between the air delivered on the two sides of the windshield could under some circumstances involve insufficient de-icing or de-emisting taking place, or could even give rise to a situation in which de-emisting or de-icing occurs only on one half of the windshield.